The present invention relates to a process for preparing a catalyst support for (stereospecific) polymerization of alpha-olefins, in particular propylene, and to the support thus obtained.
Alpha-olefin polymerization in general is carried out using Ziegler-Natta type catalysts. The Ziegler-Natta type catalyst system is generally constituted by two non-dissociable elements: a transition metal-based catalytic component deposited on a magnesium chloride-based support and a co-catalyst generally based on an aluminum compound. Numerous patents describe these catalyst components and their supports.
European patent application EP-A-0,239,475 discloses a controlled morphology catalyst spherical support for alpha-olefin polymerization. The catalyst support is obtained by reaction a chlorine-containing organic compound in the presence of an electron donor and a prior mixture of an alkylmagnesien and an aluminoxane and/or aluminosiloxane and, optionally an electron donor. The support is then subjected to an activation step using a chlorine-containing compound prior to impregnation by a transition metal halide. This process leads to effective catalysts for ethylene polymerization. They have however proved to be less effective in particular for polymerization of propylene.
Activation of catalyst supports by treatment with cyclic monoethers for ethylene polymerization catalysts is known. European patent application 0,554,141 discloses, for example, a process for activating a magnesium chloride-based support that enters into the manufacture of the ethylene catalytic polymerization component. This process comprises activating the support in suspension in an inert liquid using a cyclic mono-ether. That patent does nevertheless not disclose nor suggest the possibility of such activation for controlled morphology catalyst supports adapted to propylene polymerization.
U.S. Pat. No. 3,642,746 discloses a Ziegler-Natta catalyst system useful to reduce the ash content of the obtained polymer. The catalyst is prepared by pretreatment of a divalent metal dihalide with en electron donor and impregnation of the obtained support with a transition metal halide.
A process for polymerization of ethylene in the gas phase leading to a linear polyethylene with a narrow weight distribution is known from U.S. Pat. No. 5,055,535. This process uses a Ziegler-Natta catalyst in presence of an alkylaluminium and a monoether. According to this document, the monoether should not be in contact with the catalyst in absence of the cocatalyst in the medium. The monoether constitues therefore an external Lewis Base and intervenes only during the polymerization. Further the monoether does not act as a polymerization activator and does therefore not allow to enhance productivity.
It is known that stereospecific polymerization of xcex1-olefins beyond ethylene such as propylene requires a stereospecific type catalyst. Indeed, contrary to polymerization of ethylene, which is a symmetrical molecule, the polymerization of an asymmetric xcex1-olefin, such as propylene, can lead to isotactic, syndiotactic or atactic chaining. The use of a stereospecific catalyst can then ensure that polymers of the desired structure, such as predominantly syndiotactic or isotactic, for example, are obtained. This explains why catalysts employed for ethylene polymerization are not necessarily suitable for polymerizing polypropylene.
It has now been surprisingly found that activation adapted to ethylene can be applied to a support adapted to propylene.
The invention makes it possible to obtain catalysts which are both highly effective and highly stereospecific for the polymerization of xcex1-olefins with at least 3 carbon atoms, in particular propylene.
The invention consequently pros ides a process for preparing a catalyst support for polymerizing xcex1-olefins comprising the steps of:
(i) reacting, in the presence of a first electron donor, a chlorine-containing organic compound and a prior mixture of an alkylmagnesien and an aluminoxane and/or aluminosiloxane and/or alkylaluminum and, optionally, a second electron donor; and
(ii) activating the product from step (i) in suspension in an inert liquid by means of an activation electron donor.
The invention also provides a catalyst support obtainable by the process according to the invention.